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			<h1>common_schema</h1> <strong>2.2</strong> documentation
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				<h2><a href="query_script_while.html">query_script_while</a></h2>	

<p>
QueryScript Flow Control: <strong>while</strong> statement
</p>

<h3>SYNOPSIS</h3>
<p>
<blockquote><pre>while (expression)
  statement;
[otherwise
  statement;]
</pre></blockquote>
</p>

<h3>DESCRIPTION</h3>
<p> 
<i>while</i> is a flow control looping structure. It makes for a condition test based on
a given <a href="query_script_expressions.html">expression</a>. 
</p>
<p>
As long as the <i>expression</i> holds true, the statement (or block of statements) following 
the <i>while</i> statement are executed.
The expression is evaluated before each iteration of the loop.
</p>
<p>
The <i>while</i> loop terminates in the following cases:
<ul>
	<li>The <i>expression</i> does not hold true</li>
	<li>A <a href="query_script_break.html">break</a> statement is executed inside the loop (but not inside a nested loop)</li>
	<li>A <a href="query_script_return.html">return</a> statement is executed inside the loop</li>
	<li>An uncaught error is raised (at the moment all errors are by default uncaught unless handled in user code)</li>
</ul>
</p>
<p>
	The <strong>otherwise</strong> clause is optional. It will execute once should no iteration
	take place. That is, if at least one <strong>while</strong> iteration executes,
	<strong>otherwise</strong> is skipped.
</p>
<p>
Empty statements are not allowed in QueryScript. However, empty blocks are, and the 
<i>while</i> clause may be followed by an empty block, or by the do-nothing <a href="query_script_pass.html">pass</a> statement.
</p>


<h3>EXAMPLES</h3>
<p>
Remove leading digits from text:
<blockquote><pre>
set @txt := '12864xyz';
while(left(@txt,1) in ('0','1','2','3','4','5','6','7','8','9'))
  set @txt := substring(@txt, 2);
  
SELECT @txt;
  
+------+
| @txt |
+------+
| xyz  |
+------+  
</pre></blockquote>
</p>

<p>
<strong>DELETE</strong> statement as <i>expression</i>:
<blockquote><pre>
while (DELETE FROM world.Country WHERE Continent = 'Asia' LIMIT 10)
{
  -- We remove 10 rows at a time, and throttle by waiting in between 
  -- deletions twice the amount of time executed on deletion.
  throttle 2;
}
</pre></blockquote>
</p>


<p>
Repeatedly issue a query for <strong>60</strong> seconds. Output number of repetitions:
<blockquote><pre>
set @counter := 0;
while(script_runtime() < 60) 
{ 
  SELECT Continent, COUNT(*) FROM world.Country GROUP BY Continent;
  set @counter := @counter + 1;
}
SELECT @counter;

+----------+
| @counter |
+----------+
|    15654 |
+----------+
</pre></blockquote>
</p>

<p>
Use an <strong>otherwise</strong> clause:
<blockquote><pre>
set @count := 0;
while (@count > 22)
  set @count := @count-1;
otherwise
  echo 'not a single iteration';

+--------------------------+
| echo                     |
+--------------------------+
| 'not a single iteration' |
+--------------------------+
</pre></blockquote>
</p>


<h3>SEE ALSO</h3>
<a href="query_script_if_else.html">if-else</a>,
<a href="query_script_loop_while.html">loop-while</a>,
<a href="query_script_foreach.html">foreach</a>,
<a href="query_script_expressions.html">expressions</a>,
<a href="query_script_break.html">break</a>

<h3>AUTHOR</h3>
Shlomi Noach
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